A number of techniques exist for the elemental analysis of objects using X-rays. Some of these techniques rely on the different X-ray attenuation characteristics of elements, whereas others rely on X-ray fluorescence.
An example of an attenuation-based analysis technique utilizes characteristic elemental resonance energies. The attenuation of an X-ray beam of a sufficiently narrow spectral bandwidth increases substantially, when the central energy increases over the resonance energy of a constituent element of a test object. X-ray microscopes taking advantage of this characteristic have been developed for element-specific imaging. The microscopes typically combine a source, such as a synchrotron, a monochromator, a lens, such as a zone plate lens, a detector array, and possibly a scintillator to generate an image of a given test object. Typically, the microscopes are used in transmission. Two images at X-ray energies below and above the resonance energy of the element of interest are often required to obtain the necessary contrast between the element of interest and other constituent elements of the test object to thereby yield an image of the element's distribution within the test object.
X-ray fluorescence analysis or spectrometry (XRF) is a nondestructive analysis technique, which uses primary radiation, such as X-rays or energetic electrons, to eject inner-shell electrons from the atoms of the test object, yielding electron vacancies in the inner shells. When outer-shell electrons in the atoms fill the vacancies, secondary radiation is emitted with energies equal to the energy difference between the inner- and outer-shell electron states. The fluorescence emissions are characteristic of different elements. Thus, measurement of the spectrum of the secondary X-rays yields a quantitative measure of the relative abundance of each element that is present in the test sample.
Element-specific imaging of a test object with a spatial resolution better than about 1 micrometer is obtained currently by analyzing the X-ray fluorescence spectrum at each point by raster scanning a small probe of ionizing radiation, such as X-rays or energetic electrons, across the test object. Element specific imaging with a spatial resolution approaching 100 nanometers (nm) has been demonstrated with high elemental sensitivity using a high brightness synchrotron radiation source, but the serial nature of the raster scanning significantly limits the throughput and the high source brightness requirement makes it unpractical for producing an element-specific imaging system using a laboratory x-ray source.